This invention relates to the melt preparation of polycarbonate using a two stage process in which a diaryl carbonate and dihydroxy aromatic compound are reacted together in a first reaction stage to provide a polycarbonate oligomer and thereafter, in a second stage, the polycarbonate oligomer is converted into a product polycarbonate. The process employs a tetraalkyl phosphonium carboxylate as a catalyst in the first reaction stage. An alkali metal hydroxide is added as a co-catalyst in the second reaction stage. The method provides a highly endcapped product polycarbonate comprising a lower level of Fries product than is provided by other known methods.
Increasingly, polycarbonate is being prepared by the melt reaction of a diaryl carbonate with a dihydroxy aromatic compound in the presence of a transesterification catalyst, such as sodium hydroxide. In this xe2x80x9cmeltxe2x80x9d process, reactants are introduced into a reactor capable of stirring a viscous polycarbonate melt at temperatures in excess of 300xc2x0 C. Typically, the reaction is run at reduced pressure to facilitate the removal of by-product aromatic hydroxy compound formed as the diaryl carbonate reacts with the dihydroxy aromatic compound and growing polymer chains. It is frequently desirable to prepare polycarbonates having a high level of endcapped polymer chain ends in order to promote polymer stability and to reduce the tendency to accumulate a static charge of molded articles prepared from the polycarbonate. Thus, it is desirable to maximize the percentage of polymer chains terminating with aryloxy groups, the xe2x80x9cendcappedxe2x80x9d chains, while minimizing the percentage of polymer chains terminating with hydroxyl groups.
The Fries rearrangement is a ubiquitous side reaction taking place during the preparation of polycarbonate using the xe2x80x9cmeltxe2x80x9d process. The resultant xe2x80x9cFries productxe2x80x9d serves as a site for branching of the polycarbonate chains thereby affecting flow and other properties of the polycarbonate. Although, a low level of Fries product may be tolerated in the product polycarbonate produced by the melt process, the presence of higher levels of Fries product may negatively impact performance characteristics of the polycarbonate such as moldability and toughness. Thus, melt polymerization methodology useful for the preparation of polycarbonate in which the formation of Fries product has been minimized represents a long sought goal among those wishing to practice such methodology.
It would be a significant advantage to prepare polycarbonate by a melt polymerization method which provides both a high level of endcapping and while minimizing the amount of Fries product formation.